In a wireless communication system, a plurality of base stations are deployed, and each of the base stations communicates with a terminal within the communication area of the base station itself. This communication area is called a cell.
In the wireless communication system, in order to increase the number of terminals, to which the base station simultaneously communicates, the cell can be divided by providing the transmission antenna of the base station with directivity. The division of the cell is called a sector.
When a plurality of cells use the same frequency, the communication quality of the cells may be degraded by interference from adjacent cells.
Similarly, when a plurality of sectors use the same frequency, the communication quality of the sectors may be degraded by interference from adjacent sectors.
It is generally considered that the interference from an adjacent sector is sufficiently reduced by directivity of the transmission antenna. But in fact, if the sectors are at the same distance from the base station, the sectors have equal interference without regard to the directivity; therefore, the interference is not reduced and degradation of the communication quality is not removed.
As a technique for reducing the interference from an adjacent cell, a technique for reducing the interference from the adjacent cell in a downlink of the OFDMA (Orthogonal Frequency Division Multiple Access) is described in Patent Document 1, for example.
According to the technique described in Patent Document 1, a central entity connected to a plurality of base stations allocates a frequency group to each base station, and further allocates the frequency group of each base station to terminals in a cell. The allocation of wireless resources (frequency) is called ‘scheduling.’ Each terminal measures the reception power of a pilot signal from each base station, and the central entity does the scheduling based on the reception power reported from each terminal. When a terminal is located at the cell boundary within a cell of a base station, the adjacent cell transmits signals with reduced transmission power uniformly toward the terminals in the cell of itself, to which the same frequency group as that of the terminal is allocated.
With the technique described in Patent Document 1 applied to the adjacent sector, when a terminal is located at the sector boundary within a sector, a base station will transmit signals with reduced transmission power uniformly toward the terminals in the adjacent sector, to which the same frequency group as that of the terminal is allocated.
Here, the communication area of the base station is largely divided into a multicast area for transmitting the same signal to a plurality of terminals and a unicast area for transmitting a signal unique to a terminal.
When the minimum unit of the communication area of the base station is a cell, the multicast area and the unicast area are a multicast cell and a unicast cell divided by cell, and when the minimum unit of the communication area of the base station is a sector, they are a multicast sector and a unicast sector divided by sector.
Here, it is assumed that the minimum unit of the communication area of the base station is a cell and a unicast cell is adjacent to a multicast area consisting of a plurality of multicast cells.
In the circumstances, since the signals transmitted from the multicast cells forming the multicast area are the same and since the correlation of the signals is strong, the interference from the multicast area to the unicast cell is stronger than that from other adjacent unicast cells.
Since the communication quality of the unicast cell is degraded as it is largely affected by the interference from the multicast area, the interference from the multicast area needs to be reduced to maintain the communication quality.
But as the technique described in Patent Document 1 simply decreases the transmission power toward the adjacent cells without distinguishing the unicast cell and the multicast cell, it has a problem in that the interference from the multicast area is not sufficiently reduced.
Besides, as the multicast cell continuously performs the transmission in general, the technique has a problem in that, if the transmission power of the multicast cell is always decreased, the multicast cell is always made small.